Grossular Garnet

Grossular is the calcium-aluminum end-member of the garnet group and one of the most color-diverse garnet species. Its name comes from the botanical name for gooseberry, after the pale green of some crystals.

Grossular includes several famous gem varieties: vivid green chromium/vanadium-bearing tsavorite, cinnamon-orange hessonite, mint-green Merelani garnet, colorless leuco garnet, and massive green hydrogrossular ('Transvaal jade').

Its wide color range and good hardness make grossular both an important gemstone and a favorite of mineral collectors.

Grossular forms primarily by metamorphism of impure calcareous rocks — limestones and marls altered during contact or regional metamorphism — and in skarns where calcium-rich rocks react with silica-bearing fluids.

The green tsavorite variety develops in graphitic gneisses where trace vanadium or chromium is present, while hessonite occurs in calc-silicate gneisses and gem gravels.

Major sources include Kenya and Tanzania (tsavorite, Merelani mint), Sri Lanka and India (hessonite), Mexico, Russia, Canada, and Pakistan.

Grossular is recognized by its garnet crystal habit (often dodecahedra), vitreous luster, and isometric, singly refractive character.

• Hardness: 6.5-7.5.
• Streak: white.
• No cleavage; conchoidal to uneven fracture.
• No pleochroism — a key test versus emerald and tourmaline.

Green grossular (tsavorite) can resemble emerald but is singly refractive and lacks emerald's flaky inclusions. Hessonite shows characteristic treacle inclusions. Hydrogrossular is massive and translucent, resembling jade but denser and singly refractive.

Gem-quality grossular is faceted into fine jewelry — tsavorite in particular rivals emerald and commands high prices. Hessonite, mint garnet, and leuco garnet are also cut for collectors and jewelry.

Massive hydrogrossular is carved and cabbed as a jade substitute ('Transvaal jade').

Grossular has limited industrial use; its value lies in gemstones and mineral specimens. Metaphysical lore associates it with abundance and healing, a traditional belief.